1. Field of the Invention
The invention generally relates to the optimization of electronic fuses, and more particularly to a method and apparatus for an electronic fuse polysilicon resistor for high current applications and increase resistance to ESD (electrostatic discharge) failure.
2. Background Description
Optimization of a polysilicon electronic fuse element is important for fuse initiation and verification of such initiation, and prevention of failure from ESD events. ESD events can lead to destructive failure of fuse elements.
FIG. 1 illustrates an example of a related art electronic fuse resistor 10 having a salicided polysilicon film 14 overlying a polysilicon film 12. The related art fuse 10 also includes metal contacts 16 in electrical communication with the salicided polysilicon film 14. To maintain a low resistance, the related art electronic fuse 10 is of a relatively narrow width.
FIG. 2 illustrates a cross-section of the related art electronic fuse 10 along line A-A′ of FIG. 1. As can be seen in the cross-section, the polysilicon film 12 and the salicided polysilicon film 14 of the related art fuse 10 are about the same width. Furthermore, the salicided polysilicon film 14 forms a single continuous conductor providing a single current flow path which is distributed over the entire the surface of the polysilicon film 12.
In the related art fuses, the width of the salicided polysilicon film 14 is the same as the width of the polysilicon film 12. Consequently, as the polysilicon film 12 is made larger to withstand larger currents, the salicided polysilicon film 14 becomes larger and requires higher current loads to blow. Conversely, as the salicided polysilicon film 14 is reduced in size to blow at smaller currents, the polysilicon film 12 becomes more susceptible to damage.
As a result, the window in which to blow the salicide film 14 and maintain the integrity of the insulator 12 and related polysilicon line is narrow. In other words, there is a small difference between the minimum current necessary to blow the fuse and the amount of current which will damage the insulator supporting the salicided film 12. As such, the current pulse width to implement the fuse blow is limited to a relatively narrow given time and current domain. Accordingly, a blown fuse may be accompanied by a damaged insulator impairing functioning of the associated circuit.
At electric currents above the critical current-to-failure, the related art polysilicon fuse resistor structure can lead to metal blistering, extrusion and melting. High current flow through the structure of related art fuses can lead to cracking of the insulator films due to high thermal and mechanical stress. Thermal gradients in the surrounding insulator which may lead to mechanical stresses which exceed the yield stress can lead to insulator cracking. Such cracking can cause loss of integrity of the dielectric and semiconductor chip. Accordingly, related art fuses may malfunction when blowing upon the application of high currents.
Hence, because related art fuses are susceptible to damage due to high currents, a structure which can sustain high currents and maintain structural integrity and yet lead to fuse initiation and removal of the salicide during the fuse initiation is needed.